feat: ConfigRegistry + SettingsAPI for plugin config access

Core exposes a unified settings interface through Plugin SDK:
- internal/config/registry.go: thread-safe namespaced KV store
  - Register/Get/Set/List/Delete with JSON persistence
  - Flush() for explicit save to disk
- SettingsAPI in SDK: plugins call plugin.Settings().Get/Set/List
  - Full access: read/write any key (core.*, plugin.<name>.*)
- plugin.Registry.SetConfigRegistry() wires it into SDK plugins
- main.go registers core.* keys at startup, flushes on shutdown
- 7 tests for ConfigRegistry

internal/config/registry_test.go

@@ -0,0 +1,105 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestRegistryBasic(t *testing.T) {
+	r := NewConfigRegistry("")
+	r.Register("core.llm.model", "deepseek-v4-flash")
+	r.Register("plugin.qq.access_token", "abc123")
+
+	val, err := r.Get("core.llm.model")
+	if err != nil {
+		t.Fatalf("Get error: %v", err)
+	}
+	if v, ok := val.(string); !ok || v != "deepseek-v4-flash" {
+		t.Fatalf("expected deepseek-v4-flash, got %v", val)
+	}
+
+	keys := r.List("core")
+	if len(keys) != 1 || keys[0] != "core.llm.model" {
+		t.Fatalf("expected [core.llm.model], got %v", keys)
+	}
+
+	r.Set("core.llm.model", "gpt-4")
+	val, _ = r.Get("core.llm.model")
+	if v, _ := val.(string); v != "gpt-4" {
+		t.Fatalf("expected gpt-4, got %v", val)
+	}
+}
+
+func TestRegistryPersist(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "settings.json")
+
+	r := NewConfigRegistry(path)
+	r.Register("core.log_level", "debug")
+	r.Set("plugin.test.key", 42)
+	if err := r.Flush(); err != nil {
+		t.Fatalf("Flush: %v", err)
+	}
+
+	r2 := NewConfigRegistry(path)
+	val, err := r2.Get("plugin.test.key")
+	if err != nil {
+		t.Fatalf("Get after reload: %v", err)
+	}
+	if v, _ := val.(float64); v != 42 {
+		t.Fatalf("expected 42, got %v", val)
+	}
+}
+
+func TestRegistryDelete(t *testing.T) {
+	r := NewConfigRegistry("")
+	r.Register("a.b", "1")
+	r.Register("a.c", "2")
+	r.Delete("a.b")
+	keys := r.List("a")
+	if len(keys) != 1 || keys[0] != "a.c" {
+		t.Fatalf("expected [a.c], got %v", keys)
+	}
+}
+
+func TestRegistryDump(t *testing.T) {
+	r := NewConfigRegistry("")
+	r.Register("x", 1)
+	r.Register("y", "two")
+	dump := r.Dump()
+	if len(dump) != 2 {
+		t.Fatalf("expected 2 keys, got %d", len(dump))
+	}
+}
+
+func TestRegistryUnknownKey(t *testing.T) {
+	r := NewConfigRegistry("")
+	_, err := r.Get("nonexistent")
+	if err == nil {
+		t.Fatal("expected error for unknown key")
+	}
+}
+
+func TestRegistryFlush(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "settings.json")
+	r := NewConfigRegistry(path)
+	r.Set("k", "v")
+	if err := r.Flush(); err != nil {
+		t.Fatalf("Flush: %v", err)
+	}
+	data, _ := os.ReadFile(path)
+	if len(data) == 0 {
+		t.Fatal("expected persisted data")
+	}
+}
+
+func TestRegistryFlushIdempotent(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "settings.json")
+	r := NewConfigRegistry(path)
+	r.Set("k", "v")
+	r.Flush()
+	r.Flush() // second flush should not error
+}